The following references are considered to be pertinent for the purpose of understanding the background of the present invention:    (1) Tenne, R. Nature Nanotech. 2006, 1, 103.    (2) Tenne, R, Margulis, L., Genut M. & Hodes, G. Nature 1992, 360, 444.    (3) Feldman, Y., Wasserman, E., Srolovitz D. J. & Tenne R. Science 1995, 267, 222.    (4) (a) Deepak, F. L.; Margolin, A.; Wiesel, I.; Bar-Sadan, M.; Popovitz-Biro, R.; Tenne, R. Nano 2006, 1, 167.    (b) Etzkorn, J.; Therese, H. A.; Rocker, F.; Zink, N.; Kolb, Ute.; Tremel, W. Adv. Mater. 2005, 17, 2372.    (5) Zak, A.; Feldman, Y.; Lyakhovitskaya, V.; Leitus, G.; Popovitz-Biro, R.; Wachtel, E.; Cohen, H.; Reich, S.; Tenne, R. J. Am. Chem. Soc. 2002, 124, 4747.    (6) Ivanovskaya, V. V.; Heine, T.; Gemming S.; Seifert, G. Phys. Stat. Sol. B: Basic Solid State Physics 2006, 243, 1757.    (7) Schuffenhauer, C.; Popovitz-Biro R.; Tenne, R. J. Mater. Chem. 2002, 12, 1587.    (8) Schuffenhauer, C.; Parkinson, B. A.; Jin-Phillipp, N.Y.; Joly-Pottuz, L.; Martin, J.-M.; Popovitz-Biro R.; Tenne, R. Small 2005, 1, 1100.    (9) Margolin, A.; Popovitz-Biro, R.; Albu-Yaron, A.; Rapoport L.; Tenne, R. Chem. Phys. Lett. 2005, 411, 162.    (10) Seifert, G.; Köhller, T.; Tenne, R. J. Phys. Chem. B. 2002, 106, 2497.    (11) Scheffer, L.; Rosentzveig, R.; Margolin, A.; Popovitz-Biro, R.; Seifert, G.; Cohen, S. R.; Tenne, R. Phys. Chem. Chem. Phys. 2002, 4, 2095.    (12) Yang, D.; Frindt, R. F. Mol. Liq. Cryst., 1994, 244, 355;    (13) (a) Zhu, Y. Q.; Hsu, W. K.; Terrones, M.; Firth, S.; Grobert, N.; Clark, R. J. H.; Kroto H. W.; Walton, D. R. M. Chem. Commun. 2001, 121;    (b) Hsu, W. K.; Zhu, Y. Q.; Yao, N.; Firth, S.; Clark, R. J. H.; Kroto H. W.; Walton, D. R. M. Adv. Fund. Mater. 2001, 11, 69;    (c) Nath, M.; Mukhopadhyay, K.; Rao, C. N. R. Chem. Phys. Lett. 2002, 352, 163;    (14) K. S. Coleman, J. Sloan, N. A. Hanson, G. Brown, G. P. Clancy, M. Terrones, H. Terrones and M. L. H. Green, J. Am. Chem. Soc. 2002, 124, 11580.    (15) M. Brorson, T. W. Hansen, and C. J. H. Jacobsen, J. Am. Chem. Soc. 2002, 124, 11582.    (16) K. K Tiong, T. S. Shou and C. H. Ho, J. Phys. Condens. Matter. 2000, 12, 3441.    (17) K. Biswas, C. N. R. Rao J. phys. Chem. B 2006 110, 842.    (18) Y. Feldman et al., Solid State Sci., 2, 663 (2000).
MoS2 and WS2 are quasi two dimensional (2D) compounds. Atoms within a layer are bound by strong covalent forces, while individual layers are held together by van der Waals (vdW) interactions. The stacking sequence of the layers can lead to the formation of a hexagonal polymorph with two layers in the unit cell (2H), rhombohedral to with three layers (3R), or trigonal with one layer (1T). The weak interlayer vdW interactions offer the possibility of introducing foreign atoms or molecules between the layers via intercalation. Furthermore, MoS2, WS2 and a plethora of other 2D compounds are known to form closed cage structures which are referred to as inorganic fullerene-like (IF) and inorganic nanotubes (INT), analogous to structures formed from carbon [1]. One of the initial methods of synthesis of IF-MoS2 and IF-WS2 involved starting from the respective oxide nanoparticles [2, 3]. Subsequently synthesis of IF-NbS2 and IF-MoS2 using a gas-phase reaction starting from MoCl5 and NbCl5, respectively, and H2S has been demonstrated [4a, 7]. A similar strategy for the synthesis of IF-MoS2 nanoparticles using the gas phase reaction between Mo(CO)6 and sulfur, has been reported [4b]. The two kinds of reactions progress along very different paths, which has a large effect on the topology of the closed-cage nanoparticles. The conversion of the metal-oxide nanoparticles to sulfides (IF) starts on the surface of the nanoparticles progressing gradually inwards in a slow diffusion-controlled fashion. Contrarily, the gas-phase reaction proceeds by a nucleation and growth mode starting from, e.g. a small MoS2 nuclei and progressing outwards rather rapidly.
Modification of the electronic properties of layered-type semiconductors can be accomplished either by intercalation of foreign atoms in the host lattice, or by doping/alloying process of the semiconductor. In the intercalation process alkali or another moiety like amine diffuses into the van der Waals gap between each two layers. Once it resides in the proper site it donates its valence electron to the host lattice making it n-type conductor. In the case of doping and alloying the metal atoms go into the layer itself substituting the host transition metal atom. If the substituting atom (e.g. Nb) has one less electron in its outer shell than the host metal atom (Mo), the lattice becomes p-doped. If the substituting metal atom has one extra electron (Re), the lattice becomes n-type. Doping is usually limited to below 1% substitution. In the case of alloying, the guest atoms come in significant concentrations (>1%). If the percolation limit is surpassed (e.g. Mo0.75Nb0.25S2) the lattice becomes essentially metallic.
Following the successful synthesis of the IF nanoparticles and inorganic nanotubes, foreign atoms have been incorporated into their lattice by intercalation of IF nanoparticles. For instance, IF nanoparticles of MoS2 and WS2 were intercalated by exposure to alkali metal (potassium and sodium) vapor using a two-zone transport method [5]. Alloying or doping of inorganic nanotubes has been reported for specific cases of Ti-doped MoS2 nanotubes, Nb-doped WS2 nanotubes [13(a),(b)]. In addition, W-alloyed MoS2 nanotubes have been synthesized by varying the W:Mo ratio [13(c)].
The effect of Nb substitution on the electronic structure of MoS2 was investigated theoretically using density functional tight binding method (DFTB) [6]. However, no scientific and experimental confirmation for the control of electrical properties of either nanotubes or fullerene-like nanoparticles by alloying/doping was reported. The intercalation in these compounds is mediated by their structure and can bring about significant changes in their structure and their physical properties. By varying the intercalant and its concentration, a large number of compounds with different properties can be prepared. The intercalation reaction is generally accompanied by charge transfer between the intercalating species and the host layer, which serves as the driving force for the intercalation reaction. The transition metal dichalcogenides only form intercalation complexes with electron donor species, so the process here is of electron transfer from the guest moiety to the host lattice. Such process can be used to ‘fine tune’ the electronic properties of the host material in a controllable way. It is thus possible to achieve semiconductor-to-metal transitions with intercalation. It must be born in mind though that the intercalated nanoparticles are very sensitive to the ambient atmosphere and generally loose their unique electrical properties after short exposure to the atmosphere.